Incremental BlockEncoder API

[Gozala]

One of the problems we keep running into is: does this data going to fit the block size limit ?

Right now there is no great solution for this, only thing we can do try and encode bunch of times and measure the size. Unfortunately that is not really a great option, because:

1. Previously encoded data is not reused, so we waste computation and create more data to be GC-ed.
2. If we are going above the block size limit there is no good way to backtrack

I do not know what the answer is here, but I do like the way CARBufferWriter came out and I think maybe something along the same lines could work here as well. Specifically I would like to:

1. Allocate and pass in buffer to encode data into as opposed to just buffer out for the encoded node.
   • This also provides better control in cases where we want to encode several things into a larger buffer.
   • It also implies you can't accidentally create a block size which is greater than block size limit.
2. Ideally API should allow encoder avoid re-encoding same data over and over again.